The effects of dopamine and serotonin on rat dorsal root ganglion neurons: an intracellular study.

The effects of bath application of dopamine and serotonin (10(-10)-10(-8) M) were studied in the superfused dorsal root ganglia of 30-36-day-old rats by means of the intracellular technique. In the majority of cells, dopamine and serotonin caused depolarization (60% and 64% of the tested cells, respectively). In other cells hyperpolarization, biphasic reactions or absence of responses have been observed. All reactions were dose dependent and reversible. Depolarization was accompanied by a decrease of input membrane resistance and hyperpolarization by its increase. Some cells did not show these alterations. Monoamines were also capable of modulating spikes. In some cases dopamine (10(-8)-10(-7) M) decreased the amplitude of the action potential and increased its duration, but the same concentration of serotonin produced the opposite effect on these parameters. The correlation between the electrophysiological properties of the dorsal root ganglion neurons and their responses to monoamines were discovered. Neurons with high input membrane resistance, prolonged action potential and slow conduction velocity (small cells) were influenced much more by monoamines than neurons with low input membrane resistance, "fast" action potential and rapid conduction velocity (large cells). (1) Small cells had lower threshold to monoamines (10(-8)-10(7) M) than large ones, some of which did not respond even to 10(-6) M. (2) The amplitude and duration of monoamine-induced depolarization in small cells were on average about two to three times higher than those in large cells. These data provide evidence for the modulatory role of monoamines in spinal afferent sensory functions.